Add support for SecretTypeTLS secrets (#2)

Add support for SecretTypeTLS secrets

Kubernetes knows specific TLS secrets (SecretTypeTLS). This kind of
secrets has specifically named fields (tls.crt, tls.key, ca.crt)
different from those supported by the etcd-operator. Such secretes
are e.g. generated by the cert-manager. This change adds support for
such secrets.